1. Technical Field
The present invention relates to a display device operating in a transverse-electric-field mode such as a fringe-field-switching (hereinafter, referred to as an “FFS”) mode, a method for producing the device, and an electronic apparatus. In particular, the invention relates to a highly reliable liquid crystal display device in which in intersections of common lines and source lines, breaks in the source lines and short circuits between the common lines and the source lines are suppressed, the liquid crystal display device operating in transverse-electric-field mode, and relates to a method for producing the device and an electronic apparatus.
2. Related Art
In recent years, liquid crystal display devices have often been used in general electrical equipment as well as information and communication equipment. Liquid crystal display devices that have been commonly used each include a pair of substrates composed of, for example, glass, having electrodes and the like on surfaces of the substrates and a liquid crystal layer provided between the pair of substrates. A voltage is applied to the electrodes on the substrates to align liquid crystal molecules, thus changing light transmittance to display various images. Such liquid crystal display devices are said to operate in a longitudinal-electric-field mode. Longitudinal-electric-field liquid crystal display devices include liquid crystal display devices operating in a twisted-nematic (TN) mode and a vertical-alignment (VA) mode. These liquid crystal display devices disadvantageously have narrow viewing angle ranges. Improved longitudinal-electric-field liquid crystal display devices, e.g.[d1], multidomain-vertical-alignment (MVA) liquid crystal display devices, have thus been developed.
Unlike the foregoing longitudinal-electric-field liquid crystal display devices, liquid crystal display devices operating in a transverse-electric-field mode such as an in-plane switching (IPS) mode or the FFS mode have also been known, the transverse-electric-field liquid crystal display devices each including pairs of electrodes constituted by pixel electrodes and common electrodes only on one of the substrates.
Liquid crystal display devices operating in the IPS mode each include a pair of electrodes arranged on the same layer. An electric field is applied to a liquid crystal material in the direction substantially parallel to the substrate to align the liquid crystal molecules in the direction parallel to the substrate. Thus, such IPS-mode liquid crystal display devices advantageously have very wide viewing angle ranges compared with those of longitudinal-electric-field liquid crystal display devices. However, since IPS-mode liquid crystal display devices each include a pair of electrodes serving to apply an electric field to the liquid crystal material and being arranged on the same layer, liquid crystal molecules located above the pixel electrodes are not sufficiently driven, thus disadvantageously reducing transmittance and the like.
To overcome the foregoing problems with IPS-mode liquid crystal display devices, JP-A-2001-235763 and JP-A-2002-182230 disclose FFS-mode liquid crystal display devices. FFS-mode liquid crystal display devices each include pixel electrodes and common electrodes that apply an electric field to a liquid crystal layer, the pixel electrodes and the common electrodes being arranged as different layers that are disposed with an insulating film provided therebetween.
FFS-mode liquid crystal display devices have wide viewing angle ranges and high contrast compared with those of IPS-mode liquid crystal display devices. Furthermore, FFS-mode liquid crystal display devices can operate at lower voltages and display bright images owing to higher transmittance. Moreover, FFS-mode liquid crystal display devices have a large area of regions where pixel electrodes and common electrodes overlap in plan view, as compared with those of IPS-mode liquid crystal display devices, thereby secondarily resulting in a larger storage capacitance. This advantageously eliminates the need to separately form an auxiliary capacitance line.
An array substrate of an FFS-mode liquid crystal display device in the related art includes common electrodes (also referred to as “lower electrodes”) arranged in respective pixels and composed of a transparent conductive material, an insulating film arranged on the surface of the common electrodes and composed of, for example, silicon nitride, and pixel electrodes (also referred to as “upper electrodes”) arranged on surface portions of the insulating film corresponding to the common electrodes and composed of a transparent conductive material, the pixel electrodes each having a plurality of slits arranged in parallel with one another, and the common electrodes, the insulating film, and the pixel electrodes being arranged on a transparent substrate. The common electrodes and the pixel electrodes are composed of a transparent conductive material such as indium tin oxide (ITO) or indium zinc oxide (IZO). In such FFS-mode liquid crystal display devices in the related art, breaks or short circuits occur at positions where source lines intersect common lines or gate lines, in some cases. The breaks or short circuits occur more frequently at intersections of the source lines and the common lines than intersections of the source lines and the gate lines.
To examine the causes of these phenomena, the inventors have conducted intensive studies and found the main causes described below.
(1) The source lines and the gate lines are brought into contact with an etching solution twice. Thus, the source lines and the gate lines are excessively etched, so that surface states or cross-sectional shapes thereof change from their design states or shapes.
(2) The common electrodes are provided in respective pixels. However, the common electrodes provided in adjacent pixels are electrically connected to each other through only the common lines each having a width smaller than those of the uncovered gate electrodes, so that charges are liable to concentrate on the narrow common lines, thereby easily making a spark between each common line and a corresponding one of the source lines.
The reason the common lines and the gate lines are brought into contact with the etching solution twice is as follows. After the common lines and the gate lines are formed in parallel with one another, a film composed of a transparent conductive material is formed across the entire surface of a transparent substrate and then formed into a predetermined pattern by photolithography and etching, thereby forming the common electrodes. The common lines and the gate lines are usually composed of Mo-covered Al or an Al alloy and thus formed into predetermined patterns by wet etching. Also, the film composed of the transparent conductive material is patterned by wet etching.
The common lines and the gate lines having the predetermined patterns formed by wet etching are exposed to an etching solution again as the etching of the transparent conductive material proceeds. As a result, the gate lines and the common lines provided between adjacent pixels are brought into contact with the etching solution twice. Thus, the common lines and the gate lines are excessively etched, so that the surface states or the cross-sectional shapes thereof change from the design states or shapes. It is thus speculated that breaks of the source lines or short circuits are liable to occur at intersections between the source lines and the common lines and/or between the source lines and the gate lines.